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HMRC internal manual

Business Leasing Manual

Taxation of leases that are not long funding leases: legal expenses: timing of deduction for revenue expenditure

If the expenditure is revenue, the timing of the tax deduction will follow the charge against profits in the commercial accounts drawn up in accordance with GAAP. Therefore if that charge was spread over the primary period (which may be the case, especially if the costs are recovered by being specifically factored into the rate of interest implicit in the lease) you should follow that for tax and not allow a full deduction in the period when the costs are incurred.

If practice among leasing companies diverges on this point you might wish to seek accountancy advice.